BACKGROUND: In April 2009, a new surveillance strategy for the detection of cases of pandemic influenza (H1N1) 2009 infection and for the implementation of appropriate control measures to contain its transmission was initiated in Spain. We describe the clinical and epidemiological characteristics of confirmed cases in Spain notified by the National Epidemiological Surveillance Network from April 24 to June 30, 2009. METHODS: As part of the initial surveillance response to the pandemic, case-based clinical and epidemiological information was collected nationwide on cases under investigation for pandemic virus (H1N1) 2009 infection and their contacts. RESULTS: Of 717 confirmed cases, 91% were notified by 5 Autonomous Communities. As of June 15,49.1% of cases belonged to school outbreaks. No nosocomial outbreaks were detected. The median incubation period was 3 days. Eighty-eight percent of cases were under the age of 30 years, and 24.9% were imported. The most frequent symptoms were cough (92%) and fever (81.8%). The median duration of symptoms was 5 days. Thirteen cases required hospitalization and one died. CONCLUSIONS: During the first months of the pandemic, pandemic influenza cases experienced a mild illness similar to seasonal influenza, predominantly affecting children and young adults. By the end of June 2009, the detection of outbreaks in different settings indicated the diffusion of the pandemic virus into the community and the start of its circulation.
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